1. Field of the Invention
This invention relates to a process for preparing a vinyl chloride polymer of quality.
2. Prior Art
Vinyl chloride polymers are useful resins having excellent physical properties and find widespread application whether they are rigid or flexible.
Vinyl chloride polymers are generally molded by calendering, extrusion molding and injection molding techniques. In an advanced rigid extrusion molding technique, it is desired to increase the throughput of a molding machine. It is then desired to have a vinyl chloride polymer having a high bulk density.
In preparing vinyl chloride polymers by suspension polymerization in aqueous media, many attempts were made to produce vinyl chloride polymers having a higher bulk density, for example, by feeding an addition monomer during polymerization as disclosed in Japanese Patent Application Kokai (JP-A) No. 16800/1984, using highly saponified polyvinyl alcohol as disclosed in JP-A 7600/1982, and feeding an additional suspending agent during polymerization as disclosed in JP-A 39309/1993.
Although these processes are successful in producing polymers having a high bulk density, the polymers are less porous, leaving the problems that molded parts have many fish eyes and gelation is difficult. The low porosity also hinders monomer removal, which means that the polymer grains have a high concentration of residual unreacted monomer to adversely affect the working environment associated with polymer preparation or molding and working processes. There is a possibility that the unreacted monomer be left in molded parts, and the presence of residual monomer is detrimental in a particular application associated with food or the like.
On the other hand, improvements were made in polyvinyl alcohol as the suspending agent. There were developed oil-soluble polyvinyl alcohols having a low degree of polymerization and a low degree of saponification and modified polyvinyl alcohols having various modifying groups introduced therein. Among them, the oil-soluble polyvinyl alcohols are effective for improving monomer removal and fish eye control, but result in a lowering of bulk density and deteriorated free flow due to electrostatic charging. The modified polyvinyl alcohols fail to keep suspension systems stable with conventional techniques, leaving the problems of scale build-up and substantial particle size variation.
JP-A 311708/1992 or U.S. Pat. No. 5,342,906 proposes a dispersion polymerization method by which a vinyl chloride polymer having comparatively high bulk density, low monomer removal, good plasticizer absorption and minimized fish eyes was obtained with minimizing scale build-up in a reactor. However, the bulk density of the resulting polymer is not so fully improved from the standpoint of an extruder output in extrusion molding. Therefore, there is a demand for vinyl chloride polymers having a higher bulk density.